fix: Validate sync-branch at config-time and runtime (closes #1166) (#1168)

* fix(config): validate sync-branch at config time

Add sync-branch validation to validateYamlConfigValue() to reject
main/master at config time, preventing the validation bypass in GH#1166.

- Add case for sync-branch and sync.branch keys
- Inline validation logic to avoid import cycle (config <-> syncbranch)
- Add unit tests for rejection (main/master) and acceptance (valid names)

Part of: #1166

* fix(sync): add runtime guard for sync-branch == current-branch

Add dynamic runtime check before worktree operations to catch cases
where sync-branch matches the current branch. This provides defense
in depth for manual YAML edits, pre-fix configs, or non-main/master
branch names (trunk, develop, production, etc.).

- Check IsSyncBranchSameAsCurrent() after hasSyncBranchConfig is set
- Position check BEFORE worktree entry (CWD changes inside worktree)
- Add integration test TestSync_FailsWhenOnSyncBranch

Part of: #1166

* docs: note main/master restriction in sync-branch FAQ

Clarifies that git worktrees cannot checkout the same branch in
multiple locations, so main/master cannot be used as sync branch.

internal/config/yaml_config.go

@@ -297,6 +297,13 @@ func validateYamlConfigValue(key, value string) error {
 		if depth < 1 {
 			return fmt.Errorf("hierarchy.max-depth must be at least 1, got %d", depth)
 		}
+	case "sync-branch", "sync.branch":
+		// GH#1166: Validate sync branch name at config time
+		// Note: Cannot import syncbranch due to import cycle, so inline the validation.
+		// This mirrors syncbranch.ValidateSyncBranchName() logic.
+		if value == "main" || value == "master" {
+			return fmt.Errorf("cannot use '%s' as sync branch: git worktrees prevent checking out the same branch in multiple locations. Use a dedicated branch like 'beads-sync' instead", value)
+		}
 	}
 	return nil
 }